Check to see if the firewall flexes when you depress the clutch. The 08+ WRXs have been known to have faulty spot welds on the firewall where the pedal assembly is welded to causing this noise and will eventually get worse if not fixed. The spot welds are more an issue of safety and should be fixed by your dealer, regardless of being "flagged" since the mods have 0 to do with that manufacturer defect and can also be a safety hazard if the POS welds break.